Your team's offense huddles up, innocently enough, and chats for a few seconds about the upcoming play. They break the huddle, and suddenly the refs come in with whistles blaring and flags flying.
This could be your offense or defense. They all get lined up, but shortly before the snap, everything gets blown dead as you realize the collegians you root for can't count to 11 properly.
WHAT IS THIS CRAP?
As you know, football is an 11-on-11 game. Using more than 11 guys at a time is greatly frowned upon.
When a team is huddled up, it is not allowed to have more than 11 in that huddle. If a mistake happens and 12 guys huddle up, the idiot who forgot to sub out has three seconds to realize it and run off the field before his comrades break the huddle. Breaking the huddle with 12 men automatically gets you an illegal substitution flag.
Maybe your offense doesn't huddle, though. In that case, lining up with 12 gets the play blown dead before the snap. Defenses get a little more leeway, as they can line up with 12 without penalty, but they have three seconds to get the extra guy to run off the field. On offense or defense, if the ball is snapped while a guy obviously trying to get off the field is not yet on the sideline, that's also an illegal substitution penalty. No matter how you earn it, illegal substitution is a five yard penalty.
Of course, getting an illegal substitution penalty requires the officials to notice the 12 guys on the field before the snap. If they don't and your team runs/defends a play with 12 men, guess who pays the price? Hint: it's not the zebras.
Your team has 12 guys on the field while a play goes as normal. For one reason or another, the officials didn't catch it before the snap. During or after the play the flags come flying.
WHAT IS THIS CRAP?
This is the big brother of illegal substitution, and it doesn't like the way you're looking at it. While illegal substitution is a relatively minor five yard penalty, illegal participation gets is a 15 yard penalty.
This one more often than not gets called on the defense. Largely, it has to do with the fact that the offense gets flagged for lining up with 12, while the defense has three seconds to figure out it lined up with 12 and get the extra guy off the field. If the offense notices it first and gets off a quick snap, then it can trap the defense in this penalty. Plus offenses can be called for substitution infractions for breaking the huddle with 12, giving refs an extra chance to call the lesser foul.
There is something that seems pretty unfair about this, given that nearly every time this penalty is called, it's the refs' fault for not catching the extra guy before the play starts. That's why the NFL doesn't have illegal participation; it just has a blanket five yard penalty for having 12 guys on the field.
One last, and also very rare, case of illegal participation generally affects receivers. If an offensive player accidentally goes out of bounds on his own (not being pushed out) then comes back in bounds,
that's also illegal participation. Typically this happens in a scenario where a receiver accidentally steps out of bounds and then catches a ball. he can't touch the ball until a defender or referee has touched it. The act of going out of bounds makes him an ineligible receiver, and therefore it's illegal touching, not illegal participation (though it is illegal participation in most high school districts). The penalty is loss of down at the previous spot, the same as if the pass had been incomplete.